Mankind has for many years struggled against various insects, such as the mosquito and the black fly. These types of insects breed in various bodies of water, such as marshes, stagnant water, catch basins, ponds, woodland pools, abandoned pools and the like. Each adult female mosquito is capable of depositing hundreds of eggs, each of which experiences a complete metamorphosis. The egg hatches into a larva, and after a period of time, the larva enters into a pupa stage. The pupa stage is also spent in the water, and within a few days the adult mosquito emerges.
With respect to the commercial practices used in the field of mosquito control, and especially those intended for use primarily by governmental agencies and by large-scale industrial users, three "generations" of insecticides have been developed over the years. The first are various chemicals which are dispensed by foggers or sprayers, both on the ground and through the air. These chemicals may be classified as either adulticides or larvicides and are intended to attack and kill the adult mosquito or its larva, respectively. These chemicals usually have an inherent toxicity, which is potentially injurious to the environment, to marine life and wildlife, and ultimately to humans. As a result, these chemical insecticides have been viewed with disfavor in recent years.
One such product was "DURSBAN 10CR" produced by Dow Chemical Company around 1975. It was molded into the matrix of a polyethylene pellet, and the pellets were simply thrown into the water. Although the pellets sank to the bottom, the chemical larvicide was intended for a controlled release over a substantial duration. However, there were two problems with this product. First, it had an inherent toxicity problem which was potentially harmful to the environment. Second, and because of the long-term long-lasting effects of this product, the larvae could develop a resistance; and ultimately, the mosquitos could develop an immunity to adulticides of the same chemical family. This situation is referred to as "cross resistance" and illustrates that under adverse conditions, insects may tend to adapt. This ability to adapt, often within a few generations, can be somewhat exasperating for researchers engaged in the field of mosquito control.
As a departure from the chemical adulticides and larvicides, a second generation was developed. This second generation is known as insect growth regulators. Their purpose is to prevent the embryo from developing into an adult. The larva enters into its pupa stage but never develops into an adult. These products have very low toxicity, or practically no toxicity, and hence are not detrimental to marine life.
One such product is "ALTOSID" which is manufactured by Zoecon Corporation of Palo Alto, Calif. This product is available in two forms, one a liquid called "ALTOSID" SR-10, and the other a solid block called "ALTOSID" briquets. The active ingredient is methoprene. The methoprene is impregnated into a truncated cone-shaped charcoal briquet, and the briquets are sold as a package and are removed singly for individual use. The briquet is thrown into the water and sinks to the bottom. It decomposes slowly and is intended to have a sustained release over a period of about thirty days. During this time period, the briquets release the methoprene to prevent the mosquito larvae in the water from developing into adults. Because of the charcoal form of the briquet, it is messy and somewhat inconvenient to handle.
For about the past fifteen years, a third generation has been developed. Basically, these are bacteriological methods for spreading disease among insect populations. One of the most successful disease agents is Bacillus thuringiensis Berliner var. kurstaki, a bacterium which infects the larvae of Lepidoptera (moths) that are to be destroyed. More recently, a new variety has been uncovered for use against mosquito larvae and black fly larvae. This is Bacillus thuringiensis Berliner var. israelensis and its accompanying proteinaceous parasporal particles which contain protoxin. This bacillus is an aerobic spore-forming rod and is presently formulated into either a flowable liquid concentrate or a wettable powder, the latter being available commercially from Biochem Products under the trademark "BACTIMOS". The liquid concentrate or wettable powder is mixed with water and suspended, and the suspended particles are sufficiently fine to be sprayed homogeneously. Alternately, the Bacillus thuringiensis Berliner var. israelensis may be carried by granules of clay, sand, or other suitable granular material having a 20 to 40 mesh.
When these granules are used as a larvicidal carrier for either a chemical insecticide, an insect growth regulator, or a larvicidal micro-organism--a major problem is encountered. These granules have a specific gravity which is greater than water and thus sink towards the bottom of the water. Since the larvae breed at or near the surface of the water, much of the active larvicide is thus wasted. As a result, either the concentration of larvicide in the water is insufficient to destroy the mosquito larvae completely, or else a greater quantity of larvicide must be used to achieve the desired objective.
Moreover, when a larvicidal micro-organism of the bacillus type is used, and is sprayed on the water in the form of a liquid produced by diluting the wettable powder or liquid concentrate with water, a similar problem is encountered. The bacillus spores and protoxin particles are themselves heavier than water and sink towards the bottom. Additionally, the application of the bacillus does not have a sustained release--it is essentially "one shot"--and hence re-applications are often necessary to insure an effective mosquito control program. This is time consuming and expensive.
Besides these existing commercial practices, the prior art has disclosed various devices and methods for the control or destruction of mosquitos and other aquatic pests.
For example, the 1948 issue of MOSQUITO NEWS (vol. 9, No 2, pp. 68-71) disclosed a series of experiments conducted by Raley and Davis. These experiments and observations were made on behalf of the Consolidated Mosquito Abatement District, Fresno County, Calif. In the first experiment, a straight casting plaster was mixed with 25% emulsible DDT, the mixture was placed in a glass jar until semi-hard, and the jar was placed at the larvae source. The mixture disintegrated slowly, the DDT was not released rapidly, and this was unsatisfactory. Next, a mixture of casting plaster, sawdust, and a DDT derivative was formed into a briquet, and the briquet was placed in a perforated coffee can. This was an improvement, but still unsatisfactory, since the holes in the can became plugged in sources containing heavy organic matter. In a further step, the can was eliminated and the briquet was suspended on a wire. As reported, this worked satisfactorily in "quiet" water. However, fluctuations in the water level defeated the purpose of this fixed dispenser, and in several instances, the water level actually dropped below the suspended briquet. In an effort to correct this deficiency, the briquet was molded on a float consisting of a large wooden block. Several large-headed tacks or screws were set in the wooden block, the mold was placed around the tacks or screws, and the mixture was poured into the mold and allowed to harden.
The mixture used in the floating briquet disclosed in MOSQUITO NEWS consisted of equal parts of casting plaster and wood shavings. Apparently, this mixture (per se) would not float, or else the authors realized that sawdust will absorb moisture and eventually (if not quickly) sink towards the bottom of the body of water. Thus it was necessary to mold the mixture onto a separate layer consisting of a wood block, and to make this block substantially larger than the applied mixture in order to obtain the desired buoyancy. Moreover, the composite structure of this floating briquet would be position sensitive, that is, for maximium effect it should be carefully placed on the surface of the body of water so that the mixture containing the DDT would be below (rather than above) the wood block and hopefully remain in that position despite the turbulences in the water. Accordingly, this structure was not at all adaptable to random and convenient dispensing over various bodies of water, nor to large-scale production for commercial usage.
THE MOSQUITO NEWS publication (vintage 1948) specifically noted that the Consolidated Mosquito Abatement District consisted of a 1000 square mile area that must be regularly patrolled for mosquito sources; that this area included a vast number of small but prolific larvae hatching areas, and that it would be desirable to find a satisfactory method of larvae control other than spraying.
The general concept of floating devices or methods was also disclosed in the following U.S. Letters Patents:
No. 147,615 issued to Dayton in 1874 for an "improvement in disinfecting compounds"--this floating disinfectant consisted of sawdust dried, scorched, and treated successively with sulphate of iron and a volatile distillate of coal-tar--;
No. 2,468,394 issued to Dinsley in 1949 for a "carniverous fish repellent" in a dispersal container provided with a float composed of wood, plastic, kapuk, cork or other material; and
No. 3,127,235 issued to Benzel in 1964 for a "method of forming and maintaining films on surfaces of liquids" for various purposes, including pesticides.
Moreover, in U.S. Letters Pat. No. 3,590,119 issued to Cardarelli et al in 1971, a "floating larvicide" was disclosed consisting of a delayed-release toxic substance dissolved into a molded elastomeric matrix. Pellets of the resulting composition (preferably in vulcanized form) were dispersed upon the infested water. In U.S. Letters Pat. No. 4,228,614 issued to Cardarelli in 1980, a "floating pesticide dispenser" was disclosed consisting of a slow release floating polymer for destroying aquaeous pests, including mosquito larvae. The dispenser was in the form of a thin strip or tape of a polymer matrix, tethered to a suitable anchor, and comprising a copolymer of ethylene-vinyl acetate and/or an ethylene-propylene copolymer.
Furthermore, U.S. Letters Pat. Nos. 4,166,112 and 4,187,200 issued to Goldberg in 1979 and 1980, respectively, disclosed Bacillus thuringiensis in which a carrier was formulated as a buoyant colloidal suspension which stabilized just under the surface of the water.
According to information published by Biochem Products, a division of Salsbury Laboratories, Inc., a member of the Solvay Group, the earliest documented record of Bacillus thuringiensis was in Japan in 1901. In the decades since, at least 14 varieties of B.t have been identified from several countries on the bases of biochemical characteristics and serotyping of vegetative cell flagellar antigens. Bacillus thuringiensis, Berliner also known as HD-1, Serotype H-3a3b, or B.t. variety kurstaki, has been registered in the United States since 1961 for control of Lepidoptera larvae or caterpillars and is the type commonly used in forestry, agriculture, home and commercial gardening and horticulture. Products containing B.t reportedly have an excellent safety record with no documented incidents of serious or undesirable side effects on man and the environment. Biochem Products supplies a wettable powder or a flowable concentrate under the trademark "BACTIMOS". "BACTIMOS" is derived from B.t.i., Serotype H-14, Bacillus thuringiensis variety israelensis which was discovered in Israel in 1976. This is a larvicidal micro-organism comprising Bacillus thuringiensis Berliner var. israelensis and its accompanying proteinaceous parasporal particles which contain protoxin (commonly referred to as "B.t.i.").
For mosquito control purposes, the BACTIMOS (B.t.i.) is invariably mixed with water and is applied to large areas, using airplanes or helicopters. This method of application has been continually used in the prior art, despite the constant and critical need for an alternate delivery system for the myriad of ponds and other small bodies of water, as recognized in MOSQUITO NEWS in 1948.
Moreover, any attempt to impregnate B.t.i. (or the larvicidal micro-organism of the aforesaid Goldberg patents) into the floating thermoplastic carrier of the aforesaid Cardarelli patent, would be impractical (if not impossible) and would destroy the stated utility of these references. An exposure of the B.t.i. particles to temperatures above 70.degree. or 80.degree. Centigrade--depending upon the exposure time, which is inversely correlated with temperature--will cause the B.t.i. to suffer a protein denaturization, resulting in a change in its molecular structure and losing its activity. Thus, it would be impractical to attempt to incorporate B.t.i. into a thermoplastic or elastomeric strip of material, in view of the molding temperatures likely to be encountered. Moreover, even if the B.t.i. could be incorporated into a polymer or elastomeric matrix without substantially limiting or destroying its very efficacy, these B.t.i. particles are agglomerations of relatively-large molecules and are incapable of migrating within a polymer or elastomeric matrix. Hence, they would not even be released, since the active protein toxin has a molecular weight of approximately 28 megadaltons.
Thus despite the deficiencies and disadvantages of the devices and systems resorted to in commercial practices in the field; despite the numerous floating devices long since disclosed in the prior art for the control of various aquatic pests; despite the relatively extensive research and patent activity in the field of mosquito control; despite the early recognized and long-felt need for a commercially-practical conveniently-dispensed device or delivery system (other than spraying) for launching an effective mosquito control program for the myriad of ponds and other small bodies of water; and despite the ready availability of larvicidal micro-organisms, such as B.t.i.--no one (prior to the applicants herein) has disclosed a completely satisfactory solution to this problem of long standing.
In an effort to solve this problem of long standing, and as disclosed in the applicants' copending application, Ser. No. 466,210 filed Feb. 14, 1983, which is a continuation of application Ser. No. 333,579 filed Dec. 22, 1981, which in turn is a continuation-in-part of application Ser. No. 300,013 filed Sept. 8, 1981, a package of individual briquets was developed for mosquito control purposes. Each briquet comprised two layers suitably joined to each other. One layer comprised a plaster base containing the B.t.i. or other larvicidal micro-organism, and the other layer comprised a closed cell polymer foam. The polymer foam provided the desired buoyancy to allow the layered biquet to float freely on the surface of the water. Since the dimensions (length and width) of the external foam float were substantually coterminous with the corresponding dimensions of the plaster carrier for the larvicidal micro-organism, the layered briquet was position "neutral" or insensitive; that is, the layered briquet could be simply thrown out over a body of water without regard to the orientation of the briquet relative to the surface of the water. Since the dimensions were substantially equal, and since the plaster layer was heavier than the foam layer, the layered briquet would invariably re-orient itself so that the foam layer was on the top and the plaster layer was on the bottom, whereby the plaster layer would gradually disintegrate or be eroded away for a sustained release of the larvicidal micro-organism over a sufficient period of time for effective mosquito control purposes.
While sufficient for the purposes intended, and while constituting an improvement in the art, these layered briquets nevertheless had some characteristics which interfered with full-scale commercial production. First, the separate layers required production methods which were somewhat tedious and archaic. Second, while a free-floating application was intended, there was no means for a restrained-floating application if desired by a particular mosquito-control agency. Third, the polymer foam float was not bio-degradable and thus was not completely compatible with the surrounding environment.